1. Field of the Invention
The present invention relates to a lithium secondary battery, and, more particularly, to a battery unit having an improved structure capable of suppressing distortion of an electrode unit, and a lithium secondary battery employing the same.
2. Description of the Related Art
With the development of portable electronic devices such as cellular phones, notebook-type computers, camcorders, and the like, there has been increased research into secondary batteries that are generally capable of charging and discharging. Lithium batteries are classified into nickel-cadmium (Ni—Cd) batteries, nickel-hydride (Ni-MH) batteries, and lithium secondary batteries. Specifically, the lithium secondary batteries are being rapidly developed in view of their high operating voltage of 3.6 V or higher, which is approximately 3 times that of the nickel-cadmium (Ni—Cd) batteries or nickel-hydride (Ni-MH) batteries, and their excellent energy density per unit weight.
The lithium secondary batteries may be classified into liquid electrolyte batteries and solid electrolyte batteries, according to the electrolyte used. In general, batteries using a liquid electrolyte are referred to as lithium-ion batteries, and batteries using a polymeric electrolyte are referred to as lithium polymer batteries.
The lithium secondary batteries may be manufactured in various shapes, and are typically in cylindrical and rectangular shapes. Lithium polymer batteries, which have been given much attention recently, are manufactured in a flexible pouch shape so that they are relatively free in view of shape design. Also, lithium polymer secondary batteries, having excellent safety characteristics and being light in weight, are advantageous in attaining miniaturized and lightweight portable electronic apparatuses.
Pouch-type lithium secondary batteries are disclosed in U.S. Pat. Nos. 6,391,491 and 6,497,980.
FIG. 1 shows a conventional pouch-type lithium secondary battery 10. Referring to FIG. 1, the lithium secondary battery 10 includes an electrode unit 11 and a case 12. A space 12a, in which the electrode unit 11 is accommodated, is provided in the case 12.
The electrode unit 11 includes, although these elements are not shown, a positive electrode plate, a negative electrode plate, and a separator interposed therebetween. In a state in which the positive electrode plate, the separator, and the negative electrode plate are stacked in that order, the stacked structure is wound in a jelly-roll type structure.
Unlike in the cylindrical or rectangular lithium secondary battery, which are formed of a thick metal plate, the case 12 is formed of a thin metal foil and an insulating film laminated on both surfaces of the metal foil in a pouch type structure. A space 12a, in which the electrode unit 11 is accommodated, is provided in the case 12, and a sealing surface 12b is provided along the periphery of the space 12a for thermal fusion.
Ends of positive and negative electrode leads 13 and 14, which are electrically connected to respective electrode plates of the electrode unit 11, extend outside the sealing surface 12b. An insulating tape 15 is wrapped around a contact portion of the positive and negative electrode leads 13 and 14 and the sealing surface 12b. 
FIG. 2 is a perspective view of the electrode unit 11 shown in FIG. 1. Referring to FIG. 2, the electrode unit 11 is formed by winding a laminate structure in which the positive electrode plate, the separator, and the negative electrode plate are sequentially disposed in that order. The positive and negative electrode leads 13 and 14 extend outside the electrode unit 11 to be connected to external terminals. The insulating tape 15 is wrapped around the positive and negative electrode leads 13 and 14,
An insulating member 21 is wrapped around the outermost surface of the jelly-roll type electrode unit 11 in order to prevent the electrode unit 11 from being unwound due to its intrinsic restoring force. The insulating member 21 covers the electrode unit 11, including a winding completion portion 11a wrapped around the outer surface of the electrode unit 11, so that the shape of the electrode unit 11 is retained.
Next, the electrode unit 11 is mounted in the space 12a of the case, and the sealing surface 12b formed along the periphery of the space 12a is sealed by being thermally fused. Then, the positive and negative electrode leads 13 and 14 extending outside the case 12 are electrically connected to terminals of a protection circuit board mounted on an outer surface of the case 12.
However, the conventional lithium secondary battery 10 has some significant problems. When winding the electrode unit 11 in a state in which electrode plates having opposite polarities and a separator interposed therebetween are sequentially disposed, different tensions are applied to the respective components of the electrode unit 11 as the winding proceeds. That is, the tension applied to the positive electrode plate, the negative electrode plate and the separator is not uniform.
In such a state, if there is a change in the thickness of an electrode plate during electrolyte injection or charging/discharging, the electrode unit 11, which has a restoring force itself, deforms to restore a stable structure that can minimize the tension applied thereto.
However, since the electrode unit 11 is supported by the insulating member 21 so as to prevent positional deformation, it cannot deform freely, resulting in distortion, as indicated by a dotted line shown in FIG. 2.